Pfizer Inc said on Tuesday its vaccine to prevent infections from a bacteria that mainly spreads through hospitals and doctors’ offices and can even prove fatal, failed to meet the main goal of a late-stage study.

Pfizer announced results from the CLOVER trial (CLOstridium difficile Vaccine Efficacy TRial), a pivotal Phase 3 study evaluating its Clostridioides difficile (C. difficile)vaccine candidate (PF-06425090) in the prevention of C. difficile infection (CDI).

There are no vaccines yet to prevent the illness caused by C. diff bacterium, which has been classified as an urgent public health threat by the U.S. Centers for Disease Control and Prevention.

The infection is associated with diarrhea that may progress to a severe and debilitating illness and even result in death. Most C. diff cases occur when patients have been taking antibiotics, which wipe out friendly bacteria in the colon that normally keep C. diff under control.

CLOVER enrolled approximately 17,500 adults, 50 years and older. The study was expected to accrue 66 cases of CDI within two years of the primary vaccination series. Due to significant operational challenges created in part by the COVID-19 pandemic, the final analysis was performed at 42 cases within four years after agreement from the U.S. Food and Drug Administration (FDA) to amend the protocol.

The Pfizer vaccine only showed an efficacy of 31% in preventing the infections after the third dose, and 28.6% following the second dose in the study.

For infections recorded 14 days after the third dose, vaccine efficacy was 49% at 12 months, 47% at 24 months and 31% at final analysis.

But Pfizer said it would evaluate next steps as the trial showed the vaccine helped reduce the severity of infections.

Pfizer’s head of vaccine research and development, Kathrin Jansen, said:

We are encouraged by the promising potential benefit observed against more severe C. difficile infection, as a large portion of cases lead to extended diarrhea episodes that can require hospitalization.

Pfizer will further evaluate data from the trial and determine next steps for C. difficile vaccine program. The company plans to submit for presentation of the Phase 3 CLOVER trial results, including all secondary endpoints, at a future medical congress and for publication in a peer-reviewed scientific journal.